Method and apparatus for determining the state of charge of a lithium-ion battery

ABSTRACT

Computer-assisted methods for determining the state of charge of a specific lithium ion battery, without the need for charging and discharging the battery, by utilizing look-up tables or algorithms which store the relationships of state of charge to open-circuit voltage or to ramp-peak current, or to both for that type of specific lithium-ion battery to determine the state of charge for that specific lithium-ion battery.

FIELD OF INVENTION

[0001] This invention relates to the charging of lithium-ion batteriesand more specifically to the determination of the optimum chargingcurrents for lithium ion batteries.

BACKGROUND OF THE INVENTION

[0002] Lithium-ion secondary batteries are quickly gaining favor due totheir lightweight and high energy density. Before now, chargers werebuilt to charge the battery under relatively low currents for longperiods of time, so as to avoid battery heating and electrode damage.Alternately, a relatively high fixed current was used which was known tobe in a safe current regime for a newly manufactured battery. As thebattery ages, or if a different battery is placed on a given charger,the fixed current may exceed the safe current regime, at which timebattery heating and electrode damage might occur.

[0003] In order to save time in the charging of lithium-ion batteriesand to still assure the safety of the charging technique, it isdesirable to be able to determine an optimum charging current. Further,to improve the charging techniques it is desirable to be able to haveknowledge of the state of charge (SOC) and capacity of the battery beingcharged. Priorly in the art, SOC and capacity were obtained by fullycharging and discharging the battery in question and tracking thecorresponding battery voltage vs. energy input/output. This charging anddischarging of each battery to be charged again requires time but alsouses energy.

[0004] A need therefore exists for methods that can determine theoptimum charging current for safely yet time-effectively charging aspecific lithium-ion battery. There also exists the related need fordetermining the SOC of the battery as part of the determination of theoptimum charging current determination without the need to charge anddischarge the specific battery to be charged.

SUMMARY OF THE INVENTION

[0005] We have found that the optimal charging current of a lithium-ionbattery can be determined by performing a series of charging experimentsutilizing varying initial charging currents, and by then recording cellvoltage, cell temperature, and charging time. Specifically, we havefound that the controlling variable in determining the optimum chargingcurrent is the state of charge (SOC) of the battery. The storagecapacity of a battery is typically specified in Amp-Hours (Ah), where 1Ah=3600 coulombs, and the state of charge (SOC) of a battery is definedas the actual charge stored in a given battery capacity divided by thestorage capacity of that battery.

[0006] In accordance with one aspect of our invention, look-up tables oralgorithms for each type of lithium-ion battery are prepared and storedin a computer or database. These look-up tables co-relate eitheropen-circuit voltage vs. state of charge or ramp-peak current vs. stateof charge for each type of lithium-ion battery, or include bothco-relations. Our invention includes both the method steps for creationof the look-up tables as well as the use of the look-up tables indetermining the optimum charging current for a particular lithium-ionbattery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 shows a battery measurement circuit that is suitable foruse with one illustrative embodiment of our invention.

[0008]FIG. 2 shows a sample plot of open-circuit voltage vs. state ofcharge for a typical lithium ion battery.

[0009]FIG. 3 shows a sample plot of ramp-peak current vs. state ofcharge for a typical lithium ion battery.

DESCRIPTION OF THE INVENTION

[0010] Method 1—Open-circuit Voltage vs. State of Charge

[0011] One embodiment of our invention is a method to determine thestate of charge of a lithium-ion battery based upon the measuredopen-circuit voltage for that battery.

[0012] Referring to FIG. 1, a lithium-ion battery 100, of a known type,is shown in a measurement circuit including voltmeter 6, ammeter 5, andthermocouple 7. Power supply 3 can be used to charge battery 100 whenbattery-charging relay 4 is activated. Blocking diode 8 is used to limitthe direction of current flow so that current flows only from the powersupply 3 to the battery 100 during charging. The battery 100 can bedischarged through load 12 and blocking diode 13 whenbattery-discharging relay 11 is activated. The circuit of FIG. 1 can beused both to create the look-up tables of our invention and to determineoptimum charging current using those tables.

[0013] A computer 1 receives voltage measurements from voltmeter 6 via asignal interface 2. The computer 1 also receives battery temperaturemeasurements from thermocouple 7 and electrical current measurementsfrom ammeter 5 via the signal interface 2. The computer 1 also controlsthe on-off states of the battery charging relay 4 and thebattery-discharging relay 11 via the signal interface 2.

[0014] The computer 1 can be, for example, a Gateway Pentium computerwith a National Instrument data acquisition module installed therein.The signal interface 2 can be, for example, a National Instrumentssignal conditioning system with a digital to analog converter (DAC)module, an analog to digital converter module (ADC), a relay controlmodule, and a thermocouple module installed therein. The voltmeter 6 canbe, for example, included as a channel of the ADC module. The ammetercan be for example, comprised of a 50-amp/50-millivolt shunt and achannel of the ADC module. The power supply 3 can be, for example, aHewlett-Packard model 6032A power supply.

[0015] The circuit shown in FIG. 1 can be used to create a look-up tableof open-circuit voltage (OCV) versus state of charge (SOC) for aparticular type of lithium-ion battery as follows. First, thebattery-charging relay 4 is activated and the battery-discharging relay11 is deactivated. Next, the battery 100 is charged at an initialconstant current (I_(o)), for example 1 amp, by increasing the outputvoltage of power supply 3 while monitoring charging current into thebattery 100 using ammeter 5. The battery 100 is charged at this constantcurrent, for example 1 amp until the voltage across the battery, asmeasured by voltmeter 6 reaches a maximum permitted voltage (V_(Max)). Abattery manufacturer determines V_(Max) based on safety considerations.A typical value of V_(Max) for lithium-ion batteries is 4.2 volts percell. For a battery 100 consisting of a multiple cells connected inseries;

V _(Max) _(Battery) =V _(Max) _(Cell) *N  Eq. 1

[0016] where N is the number of cells connected in series. When V_(Max)is reached, charging is continued at this constant voltage, V_(Max), andthe charging current is reduced (float charging). When the input currentdecreases to a certain point, for example to 1% of the initial constantcurrent, the cells being charged are considered to be fully charged andat 100% SOC. The battery-charging relay 4 is then deactivated and thebattery 100 is allowed to stabilize, where battery stabilization isdetermined by the variation in the open-circuit voltage (OCV) of thebattery 100 as measured by voltmeter 6. The battery 100 is considered tobe stabilized when the rate of change of the OCV is less than athreshold, for example 0.0001 volts/minute. Stabilization time for alithium-ion battery can be about 30 minutes. The battery 100open-circuit voltage at 100% SOC (OCV_(SOC=100%)) is recorded after thebattery 100 is stabilized.

[0017] Second, the battery 100 is discharged at a predetermineddischarge rate to a lower cutoff voltage (V_(Min)) through load 12 byactivating the battery-discharging relay 11 and deactivating thebattery-charging relay 4. The predetermined discharge rate can beselected as the value to completely discharge the battery, from 100% SOCto 0% SOC, in a time ranging from between 30 minutes to 5 hours. Abattery manufacturer determines V_(Min) based on safety considerations.A typical value of V_(Min) for lithium-ion batteries is 3.0 volts percell. For a battery 100 consisting of a multiple cells connected inseries;

V _(Min) _(Battery) =V _(Min) _(Cell) *N  Eq. 2

[0018] where N is the number of cells connected in series. When V_(Min)is reached, the cells are fully discharged and at 0% SOC. Thebattery-discharging relay 11 is then deactivated and the battery 100 isallowed to stabilize, where battery stabilization is determined by thevariation in the open-circuit voltage (OCV) of the battery 100 asmeasured by voltmeter 6. The battery 100 is considered to be stabilizedwhen the rate of change of the OCV is less than a threshold, for example0.001 to 0.005 volts/minute. Stabilization time for a lithium-ionbattery can be about 30 minutes. The battery 100 open-circuit voltage at0% SOC (OCV_(SOC=0%)) is recorded after the battery 100 is stabilized.The capacity of the battery 100 can be calculated by multiplying thedischarge rate (amp) by the discharge time (hours). Note that batterycapacity is typically specified in Amp-Hours (Ah), where 1 Ah equals3600 coulombs.

[0019] Third, a predetermined number of coulombs, for example 10% of thebattery capacity, is charged (input) into the battery 100 from powersupply 3 at a predetermined charge rate by activating thebattery-charging relay 4 and deactivating the battery-discharging relay11. The predetermined charge rate can be selected as the value tocompletely charge the battery 100, from 0% SOC to 100% SOC, in a timeranging from between 30 minutes to 5 hours. The battery-charging relay 4is then deactivated and the battery 100 is allowed to stabilize, wherebattery stabilization is determined by the variation in the open-circuitvoltage (OCV) as previously described. The battery 100 open-circuitvoltage at 10% SOC (OCV_(SOC=10%)) is recorded after the battery 100 isstabilized. This procedure is repeated and a set of battery 100open-circuit voltages at various states of charge (e.g. 20%, 30% . . .90%) are recorded. In another embodiment of our invention, theopen-circuit voltage (OCV) is measured immediately at the conclusion ofeach charging interval and the measured value is extrapolated to obtainan estimate of the stabilized OCV.

[0020] Advantageously, additional tables of open-circuit voltage (OCV)versus state of charge (SOC) are prepared for various temperatures byperforming the charge-discharge-charge procedure previously describedat, for example, temperatures of −30° C., 0° C., +20° C., and +50° C.

[0021] Referring again to FIG. 1, a lithium-ion battery 100 of a knowntype, but with an unknown state of charge (SOC) is placed in ameasurement circuit consisting of voltmeter 6 with both the batterycharging relay 4 and the battery discharging relay 11 deactivated. Thevoltmeter 6 is connected through signal interface 2 to computer 1. Atechnician operating the computer 1 can input the battery type ofbattery 100 into the computer. The computer 1 will then execute an OCVcorrelation algorithm, for example a table look-up followed by linearinterpolation, to correlate the open-circuit voltage (OCV) measured bythe voltmeter with the state-of-charge for the type of battery 100 undertest. FIG. 2 shows a sample plot of open-circuit voltage vs. state ofcharge for a typical lithium ion battery.

[0022] In accordance with a further aspect of our invention, athermocouple 7 is attached to the side of battery 100 to provide batterytemperature as an input to computer 1 via signal interface 2. The OCVcorrelation algorithm will now use three inputs—battery type,open-circuit voltage, and battery temperature. For example, linearinterpolation or a similar calculation can calculate state of charge(SOC) for a battery 100 at a temperature intermediate to temperaturevalues associated with stored tables.

[0023] Method 2—Ramp-Peak Current vs. State of Charge

[0024] An alternate embodiment of our invention is a method to determinethe state of charge of a lithium-ion battery based upon the measuredramp-peak current for that battery.

[0025] The circuit shown in FIG. 1 can also be used to create look-uptable of ramp-peak current (RPC) versus state of charge (SOC) for aparticular type of lithium-ion battery as follows. First, the battery100 is brought to a known state of charge (e.g. 10% SOC) by using, forexample, the steps described in method 1 above.

[0026] Second, the battery-charging relay is activated and thebattery-discharging relay is deactivated and a monotonically increasingcurrent, for example an electrical current increasing from 0 amps to 20amps in 60 seconds, is applied to the battery 100 by the power supply 3.The dependence of current increase with time, I=f(t), can be a linearfunction and is recorded by computer 1. As the input current isincreased from 0 either 1) the input current causes the battery voltageto reach its maximum permitted voltage (V_(Max-ramp)), as measured atvoltmeter 6, or 2) the input current equals a maximum current rating ofthe power supply 3, or 3) a current limit chosen for the battery. Themaximum permitted ramp voltage can be set at 100 mV to 200 mV above themaximum permitted charge voltage, since the duration of ramp energyinput is limited. The electrical current, at which either 1) or 2),above occurs, is defined as the battery ramp-peak current (RPC). Whenthe battery 100 RPC is reached, the battery-charging relay 4 isdeactivated and the battery 100 ramp-peak current at 10% SOC(RPC_(SOC=10%)) is recorded. This procedure is repeated and a set ofbattery 100 ramp-peak currents at various states of charge (e.g. 20%,30%, . . . 100%) are recorded.

[0027] Note there is a possibility for a battery of a known type thatthere may be several low states of charge (SOC), for example 10% SOC and20% SOC, that correspond to a duplicate RPC value that is limited by thecurrent rating of the power supply 3. In this case, the rate of chargecurrent can be increased, for example from 0 amps to 20 amps in 30seconds, in order to create differing values of RPC corresponding totrue state of charge of battery 100.

[0028] Further in accordance with our invention, additional tables oframp-peak current (RPC) versus state of charge (SOC) may be prepared forvarious temperatures by performing the current ramping procedurepreviously described at, for example, temperatures of −30° C., 0° C.,+20° C., and +50° C.

[0029] Referring again to FIG. 1, a lithium-ion battery 100 of a knowntype, but with an unknown state of charge (SOC) is placed in ameasurement circuit consisting of power supply 3, ammeter 5, andvoltmeter 6 and with both the battery charging relay 4 and the batterydischarging relay 11 deactivated. The power supply 3, ammeter 5, andvoltmeter 6 are connected through signal interface 2 to computer 1. Atechnician operating the computer 1 can input the battery type ofbattery 100 into said computer. The computer 1, will then execute acontrol-loop to activate the battery-charging relay 4 and monotonicallyincrease the charging current, for example from 0 amps to 20 amps in 60seconds, in accordance with a stored look-up table of the dependence ofcurrent increase with time, I=f(t). The computer 1 increases the battery100 input current from power supply 3 until it reaches RPC as describedabove and then the computer 1 deactivates the battery-charging relay 4.The computer 1, will then execute an RPC correlation algorithm, forexample a table look-up followed by linear interpolation, to correlatethe ramp-peak current (RPC) as measured by the ammeter 5 with theramp-peak current for the type of battery 100 under test. If there is acase where the computer 100 correlation algorithm returns multiplepossible ramp-peak current (RPC) values, then the computer 1 shalladvise the user that the battery SOC is less than or equal to thehighest possible state of charge (SOC) returned by the correlationalgorithm. FIG. 3 shows a plot of RPC vs. SOC for a typical lithium-ionbattery.

[0030] As with the prior described method, the thermocouple 7 attachedto the side of battery 100 can provide battery temperature as an inputto computer 1 via signal interface 2. The RPC correlation algorithm willnow use three inputs—battery type, open-circuit voltage, and batterytemperature. For example, linear interpolation or a similar calculationcan calculate state of charge (SOC) for a battery 100 at a temperatureintermediate to temperature values associated with stored tables.

[0031] Method 3—Using Both OCV and RPC to Determine State of Charge

[0032] We have found it advantageous for the computer 1 to perform atemperature compensated open-circuit voltage (OCV) correlation algorithmas described above, followed by a temperature compensated ramp-peakcurrent (RPC) correlation algorithm, and combine, for example byaveraging, to obtain a best estimate state of charge (SOC).

What is claimed is:
 1. A method for creating a look-up table ofopen-circuit voltage versus state of charge for a particular type oflithium-ion battery and determining the capacity thereof, said methodcomprising the steps of: a) charging the battery at an initial constantcurrent until a voltage across said battery increases to a predeterminedmaximum voltage; b) continuing to charge the battery at saidpredetermined maximum voltage until an input current to said batterydecreases to a predetermined minimum current; c) measuring anopen-circuit voltage for said battery, said open-circuit voltage beingdefined as the open-circuit voltage at 100% state of charge for saidparticular type of lithium-ion battery; d) recording said open-circuitvoltage at 100% state of charge; e) discharging said battery at apredetermined discharge rate to a predetermined lower cutoff voltage andrecording a discharge time period; f) multiplying said discharge timeperiod times said predetermined discharge rate and storing the product,said multiplication product being defined as the battery capacity forsaid particular type of lithium-ion battery; g) measuring anopen-circuit voltage for said battery, said open-circuit voltage beingdefined as the open-circuit voltage at 0% state of charge for saidparticular type of lithium-ion battery; h) recording said open-circuitvoltage at 0% state of charge; i) charging said battery with apredetermined percentage of the battery capacity so that a presentbattery state of charge exceeds a previous battery state of charge; j)measuring an open-circuit voltage for said battery, said open-circuitvoltage being defined as the open-circuit voltage at the present stateof charge for said particular type of lithium-ion battery; and k)recording said open-circuit voltage at the present state of charge; l)repeating said charging said battery with a predetermined percentage ofthe battery capacity step (i), said measuring an open-circuit voltagefor said battery step (j), and said recording said open-circuit voltageat the present state of charge step (k) until the present state ofcharge of the battery equals the battery capacity for said particulartype of lithium-ion battery; and m) creating the look-up table ofopen-circuit voltage versus state of charge for state of charge valuesfrom 0% to 100%.
 2. The method in accordance with claim 1 furthercomprising the step of: a) allowing the battery to stabilize for aperiod of time, where said time period is the time required for the rateof change of the open-circuit voltage to decrease below a predefinedthreshold.
 3. The method in accordance with claim 2 wherein saidpredefined threshold for rate of change of the open-circuit voltage is0.0001 volts/minute.
 4. The method in accordance with claim 1 whereinthe predetermined percentage of the battery capacity is 10%.
 5. Themethod in accordance with claim 1 wherein all the steps of claim 1 arerepeated at a set of predetermined temperatures.
 6. The method inaccordance with claim 5 wherein the set of predetermined temperaturesconsists of −30° C., 0° C., +20° C., and +50° C.
 7. A method fordetermining the state of charge of a lithium-ion battery of a particulartype using a look-up table of open-circuit voltage versus state ofcharge for that particular type of lithium-ion battery, said methodcomprising the steps of: a) inputting a battery type into a computer; b)measuring an open-circuit voltage for said battery, said open-circuitvoltage being input into said computer; c) executing an open-circuitvoltage correlation algorithm based on inputs comprising, i) the type ofsaid battery inputted into said computer and ii) the measuredopen-circuit voltage for said battery; and d) determining the state ofcharge of the lithium-ion battery based on said open-circuit voltagecorrelation algorithm.
 8. The method in accordance with claim 7 whereinsaid open-circuit voltage correlation algorithm comprises a tablelook-up followed by linear interpolation.
 9. The method in accordancewith claim 7 wherein said inputs to said open-circuit voltagecorrelation algorithm further comprise measured battery temperature. 10.A method for creating a look-up table of ramp-peak current versus stateof charge for a particular type of lithium-ion battery, said methodcomprising the steps of: a) bringing the battery to a known presentstate of charge; b) applying a monotonically increasing current to saidbattery from a power supply, where there is a dependence of currentincrease with time; c) recording said dependence of current increasewith time; d) increasing said monotonically increasing current to saidbattery until the current reaches a value being defined as the ramp-peakcurrent at the present state of charge for said particular type oflithium-ion battery where either, i) a measured voltage for said batteryreaches a predetermined maximum voltage, or ii) said monotonicallyincreasing current equals a maximum current rating of the power supply;e) recording said ramp-peak current at the present state of charge; f)repeating said bringing the battery to a known present state of chargestep (a), said applying a monotonically increasing current step (b),said recording said dependence of current increase with time step (c),said increasing said monotonically increasing current step (d), and saidrecording said ramp-peak current at the present state of charge step (e)at a set of predetermined present states of charge for said battery; andg) creating the look-up table of ramp-peak current versus state ofcharge for the particular type of lithium-ion battery using the set ofpredetermined present states of charge for said battery.
 11. The methodin accordance with claim 10 wherein said monotonically increasingcurrent is an electrical current increasing from 0 amps to 20 amps in 60seconds.
 12. The method in accordance with claim 10 wherein the set ofpredetermined present states of charge for said battery consists of 10%,20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100%.
 13. The method inaccordance with claim 10 further comprising the steps of: a) checkingthe look-up table of ramp-peak current versus state of charge forduplicate ramp-peak current values; b) repeating the steps of claim 10for the states of charge with duplicate ramp-peak current values, exceptwherein the monotonically increasing current to said battery from apower supply, has a different dependence of current increase with time.14. The method in accordance with claim 10 wherein all the steps ofclaim 10 are repeated at a set of predetermined temperatures.
 15. Themethod in accordance with claim 14 wherein the set of predeterminedtemperatures consists of −30° C., 0° C., +20° C., and +50° C.
 16. Amethod for determining the state of charge of a lithium-ion battery of aparticular type using a look-up table of ramp-peak current versus stateof charge for that particular type of lithium-ion battery, said methodcomprising the steps of: a) inputting a battery type into a computer; b)measuring a ramp-peak current for said battery, said ramp-peak currentbeing input into said computer; c) executing a ramp-peak currentcorrelation algorithm based on inputs comprising, i) the type of saidbattery inputted into said computer and ii) the measured ramp-peakcurrent for said battery; and d) determining the state of charge of thelithium-ion battery based on said ramp-peak current correlationalgorithm.
 17. The method in accordance with claim 16 wherein saidramp-peak current correlation algorithm comprises a table look-upfollowed by linear interpolation.
 18. The method in accordance withclaim 16 wherein said inputs to said ramp-peak current correlationalgorithm further comprise measured battery temperature.
 19. A methodfor determining the state of charge of a lithium-ion battery of aparticular type using both a look-up table of open-circuit voltageversus state of charge and a look-up table of ramp-peak current versusstate of charge for that particular type of lithium-ion battery, saidmethod comprising the steps of: a) inputting a battery type into acomputer; b) measuring an open-circuit voltage for said battery, saidopen-circuit voltage being input into said computer; b) measuring aramp-peak current for said battery, said ramp-peak current being inputinto said computer; c) executing a correlation algorithm based on inputscomprising, i) the type of said battery inputted into said computer, ii)the measured open-circuit voltage for said battery, and iii) themeasured ramp-peak current for said battery; and d) determining thestate of charge of the lithium-ion battery based on said correlationalgorithm.
 20. The method in accordance with claim 19 wherein saidinputs to said correlation algorithm further comprise measured batterytemperature.